El Capitan に pip で matplotlib 入れたときのメモ。
pkg-config と freetype が入ってなかったってオチなのでそこまで内容はありません。

まずはフツーにコマンド叩く。
が、エラーが出る。

$ pip install matplotlib
Collecting matplotlib
  Using cached matplotlib-1.5.3.tar.gz
    Complete output from command python setup.py egg_info:
    IMPORTANT WARNING:
        pkg-config is not installed.
        matplotlib may not be able to find some of its dependencies
    ============================================================================
    Edit setup.cfg to change the build options

    BUILDING MATPLOTLIB
                matplotlib: yes [1.5.3]
                    python: yes [3.6.0 (v3.6.0:41df79263a11, Dec 22 2016,
                            17:23:13)  [GCC 4.2.1 (Apple Inc. build 5666) (dot
                            3)]]
                  platform: yes [darwin]

    REQUIRED DEPENDENCIES AND EXTENSIONS
                     numpy: yes [version 1.11.3]
                  dateutil: yes [dateutil was not found. It is required for date
                            axis support. pip/easy_install may attempt to
                            install it after matplotlib.]
                      pytz: yes [pytz was not found. pip will attempt to install
                            it after matplotlib.]
                    cycler: yes [cycler was not found. pip will attempt to
                            install it after matplotlib.]
                   tornado: yes [using tornado version 4.4.2]
                 pyparsing: yes [pyparsing was not found. It is required for
                            mathtext support. pip/easy_install may attempt to
                            install it after matplotlib.]
                    libagg: yes [pkg-config information for 'libagg' could not
                            be found. Using local copy.]
                  freetype: no  [The C/C++ header for freetype2 (ft2build.h)
                            could not be found.  You may need to install the
                            development package.]
                       png: yes [version 1.6.26]
                     qhull: yes [pkg-config information for 'qhull' could not be
                            found. Using local copy.]

    OPTIONAL SUBPACKAGES
               sample_data: yes [installing]
                  toolkits: yes [installing]
                     tests: yes [nose 0.11.1 or later is required to run the
                            matplotlib test suite. Please install it with pip or
                            your preferred tool to run the test suite / using
                            unittest.mock]
            toolkits_tests: yes [nose 0.11.1 or later is required to run the
                            matplotlib test suite. Please install it with pip or
                            your preferred tool to run the test suite / using
                            unittest.mock]

    OPTIONAL BACKEND EXTENSIONS
                    macosx: yes [installing, darwin]
                    qt5agg: no  [PyQt5 not found]
                    qt4agg: no  [PySide not found; PyQt4 not found]
                   gtk3agg: no  [Requires pygobject to be installed.]
                 gtk3cairo: no  [Requires cairocffi or pycairo to be installed.]
                    gtkagg: no  [Requires pygtk]
                     tkagg: yes [installing; run-time loading from Python Tcl /
                            Tk]
                     wxagg: no  [requires wxPython]
                       gtk: no  [Requires pygtk]
                       agg: yes [installing]
                     cairo: no  [cairocffi or pycairo not found]
                 windowing: no  [Microsoft Windows only]

    OPTIONAL LATEX DEPENDENCIES
                    dvipng: no
               ghostscript: no
                     latex: no
                   pdftops: no

    OPTIONAL PACKAGE DATA
                      dlls: no  [skipping due to configuration]

    ============================================================================
                            * The following required packages can not be built:
                            * freetype

    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /private/var/folders/8p/6g32nfj53nn59v0py80lzjgx2rswc_/T/pip-build-0vtpc0zd/matplotlib/

下記みたいに言われてるので両方 homebrew で入れてあげることに。

pkg-config is not installed

freetype: no  [The C/C++ header for freetype2 (ft2build.h)
                            could not be found.  You may need to install the
                            development package.]

まずは必要なものを brew コマンドでインストールした後、

$ brew install pkg-config freetype

改めて matplotlib インストール。

$ pip install matplotlib

今度はうまくいった。